L2F was designed by Cisco Systems (RFC 2341) to support the creation of secure VPDNs via tunnels over public infrastructure. The primary goal was (quoted from RFC 2341) "to divorce the location of the initial dial-up server from the location at which the dial-up protocol connection is terminated and access to the network provided."
This is primarily of interest for carriers and service providers with regard to VPDN design, aggregation, and so on. L2F has been superseded by L2TP, does not provide encryption, and is rarely used anymore. It was one of the first scalable approaches to VPDNs. L2F uses port 1701/udp and supports PPP/SLIP as encapsulated payload protocols. For an overview discussion, see the Cisco.com white papers "L2F Case Study Overview" (http://www.cisco.com/en/US/tech/tk801/tk703/technologies_design_guide_chapter09186a00800de9d6.html) and "Understanding VPDN" (http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a0080094586.shtml).